This invention relates to nonmagnetic, austenitic, stainless steels which are balanced in composition to provide high yield strength in the hot worked, forged or cold worked condition, improved resistance to galling, good resistance to intergranular stress corrosion cracking and good general corrosion resistance. The steels are particularly suited for the production of down-hole stabilizers and drill collars fabricated therefrom.
In view of the considerable depths to which the average oil fields are drilled, the requirements for the tubular alloys have changed dramatically over the years. The materials must withstand greater stresses and are required to have greater strength levels. Deeper drilling has required the use of sensitive measuring equipment to ensure the desired course of drilling is being maintained. This requires the alloy to be completely nonmagnetic to avoid any interference with the instruments. At these greater depths, the steels have encountered very aggressive chloride and sulfide environments which has required alloy modifications to improve resistance to stress corrosion cracking. The drill collars have threaded connections and must also possess reasonably good machinability. Drill collar alloys have been continuously improved for various properties while attempting to maintain the previous combination of properties because the loss of one property would make the drill collar unacceptable for use in the industry.
During drilling, the total length of drill pipe down to the drill bit must be regularly withdrawn to substitute new drill bits for the worn members. All of the pipe and drill collar joints must be threaded and disconnected many times over the entire drilling depth. The joints encounter severe conditions which contribute to galling and wear. The make-up torques at drilling sites, in most cases, are excessive and cause premature galling damage at the connections. When these drill collars are returned from the field after a journey or two into the hole (short usage), they must be repaired extensively or the damaged connections removed and new ones remachined on shortened collars. Many users employ short pieces of galling resistant beryllium-copper to minimize the joint damage. However, this alternative is very costly. In order to insure that the relatively expensive threaded drill collars can be used many times before being replaced and minimize any downtime required for making and breaking the connections, the material needs to be able to resist galling and wear.
Galling may be defined as the condition where the friction developed between two rubbing surfaces results in localized welding at the high spots on the surfaces. As more localized welding occurs during the making and breaking of the joints, the metal-to-metal contact results in the destruction of the threads which then require remachining.
The materials used for drill collars have not been modified significantly for the purpose of improving the resistance to wear and galling. This may appear quite surprising when one stops to consider that there has been a great deal of alloy development in austenitic stainless steels to improve these properties. The major explanation for the lack of development work in this area is the influence of alloy changes on the other properties required for these products.
The galling resistance of austenitic stainless steels has been related to many theories. Patents such as U.S. Pat. No. 3,912,503 have modified the surface oxide and increased the work hardening rate with a typical steel having a composition of 16% Cr, 8% Ni, 8% Mn, 4% Si, 0.08% C, 0.15% N and balance essentially iron. This alloy with good galling resistance was also designed to provide good corrosion resistance as a replacement for Type 304 stainless steel. Ni at these levels can impair stress corrosion cracking resistance.
U.S. Pat. No. 3,663,215 relies on hard silicides of Mo, Ti, V or W which are finely dispersed in the matrix to improve wear and galling. These steels have 5-12% Si, 10-22% Cr, from about 5% up to about 10% of the silicide former, 14-25% Ni, up to 0.15% C, less than 0.05% N and balance iron. However, these steels do not have adequate strength for drill collars. They also use high levels of expensive elements like Ni, Mo and W.
U.S. Pat. No. 4,146,412 has excellent galling resistance and has a broad chemistry composition of 13-19% Cr, 13-19% Ni, up to 4% Mn, 3.5-7% Si, up to 0.15% C, less than 0.04% N and balance essentially iron. These steels also have good resistance to stress corrosion cracking and chloride environments but do not have adequate strength for drill collars. Vanadium is restricted to residual amounts because of its strong ferrite forming characteristics and the added cost to balance the alloy with more nickel. Silicon and manganese were believed to lower the stacking fault energy at the planes of atom disarray within the matrix of the steel. Under loading conditions, the lower stacking fault energy promoted the development of numerous stacking faults which produced much greater strain hardening rates in the material. Silicon was believed to diffuse rapidly to points or planes of stress and thereby promote excellent galling resistance.
A standard grade which is regarded as having improved galling resistance is the straight chrome grade known as AISI Type 440C which contains about 16-18% Cr, 1% max Mn, 1% max Si, 0.75% max Mo, about 0.95-1.20% C and remainder iron. This steel is heat hardenable but has poor corrosion resistance, is magnetic and has poor formability.
From the work done previously, it is apparent that the balance between the levels of chromium, manganese, nickel, carbon, nitrogen, silicon and other elements has varied considerably.
Galling resistance in austenitic stainless steels has frequently been improved by the addition of silicon in amounts up to 5% or more. However, a close look at the alloy discussion for drill collar applications will reveal that silicon is a very strong ferrite former and this element has been typically maintained at levels below 1%. The desired composition balance for maintaining a nonmagnetic condition (a magnetic permeability below 1.02 and preferably below 1.004), requires that any increases in silicon be balanced by the addition of austenite stabilizing elements (carbon, manganese, nitrogen or nickel) and/or the reduction of the chromium. This is not an easy matter to resolve since the carbon is controlled to a very low level to avoid intergranular corrosion. Manganese is a weak austenite former but does increase the solubility limit of the alloy for nitrogen. Nitrogen is already at the highest level which can be kept in solution. Nickel is very expensive and is maintained at the lowest level possible which will preserve a low stacking fault energy and provide good resistance to stress corrosion cracking. Lowering the chromium decreases the corrosion resistance. All of these elements are balanced to provide the required levels of strength, magnetic permeability, corrosion resistance, and intergranular corrosion resistance. With all of these requirements, the industry has not made much of an attempt to change the chemistry balance to improve the problems relating to galling and wear in the threaded connections.
Applicants are aware of only two patents which address the problem of galling and wear in drill collar alloys. One is U.S. Pat. No. 4,337,088 which simply thought that any austenitic stainless having good resistance to galling (U.S. Pat. No. 3,912,503) would make a good drill collar alloy and made no changes in the composition of an existing alloy. This steel does not provide the desired level of strength required for these applications. The other austenitic stainless steel developed with good galling properties for the oil drilling applications is U.S. Pat. No. 4,840,768. This patent relates to an expensive, high nickel alloy (27-32%) having high chromium (24-28%), low nitrogen (0.015% max) and low manganese (2% max). The steel has 1.5-2.75% silicon added for improved resistance to stress corrosion cracking, but there is no relationship taught between the silicon and the galling resistance, and there is no discussion on what features of the composition balance provide the improved galling resistance. There is no teaching which relates to a low nickel, high manganese, and high nitrogen alloy with typical chromium contents for these applications and does not suggest how these elements would be balanced.
There is thus a need in the oil drilling business for an austenitic stainless steel which possesses high strength, low magnetic permeability, good corrosion resistance, good resistance to intergranular corrosion and improved resistance to galling and wear. The steels of the invention are well suited for other applications as well.